Ice detectors are known that utilize acoustic waves propagating over a distance through a structure, such as the outer material of an airplane wing, wherein the acoustic waves propagate between a transmitter transducer and a receiver transducer. Propagating waves are used in these systems to detect the presence of ice along the length of the airplane wing structure through which the waves propagate as opposed to detecting ice at a localized spot. One such ice detector as shown in U.S. Patent, Chamuel U.S. Pat. No. 4,461,178 uses propagating flexural waves that are sensitive to water and ice. Another such sensor is shown in U.S. Patent, Watkins et al. U.S. Pat. No. 4,604,612 which uses propagating shear waves that are sensitive to ice but that are insensitive to water. U.S. Patent, Rose et al. U.S. Pat. No. 5,629,485 uses propagating guided waves and extracts various features from a frequency signature of the waves for classifying contaminants such as ice, water, glycol, oils and fuel. U.S. Patent, Matuseski U.S. Pat. No. 6,378,377 uses a propagating Lamb wave which is sensitive to ice and water. In one embodiment, instead of a separate receiver the system of this patent uses a reflecting strip positioned at a distance from a transceiver transducer where the reflecting strip reflects the Lamb waves back to the transducer for analysis. Because these systems use acoustic wave energy that propagates over a substantial length of an airplane wing, the analysis of the signal, representing the acoustic wave energy picked up by the receiver transducer, to determine the presence of ice or other contaminants is extremely complex. As a result, these systems are very expensive. When a maintenance check is performed, access to the interior of the airplane wing is required to access the ice detection system. This can be extremely difficult.
It is noted that although it was believed that an acoustic wave sensor utilizing a shear wave trapped in an acoustic wave cavity, such as described in U.S. patent application Ser. No. 09/998,355 filed Nov. 20, 2001, was insensitive to water like a shear wave propagating in a plate, it has been found that sensors utilizing a trapped shear wave are sensitive to water at a level that is a multiple of ½λ, where λ is the wavelength of the acoustic wave. It is believed that this sensitivity to water is due to flexural modes, that is a mode with a vertical displacement component, generated in the acoustic wave cavity with the trapped shear wave. More specifically, because the shear wave is trapped, particles are moving faster in the interior of the acoustic wave cavity than at the edge of the cavity. This results in a “bulge” of particles that creates a vertical component in the trapped acoustic wave in addition to the transverse shear component. It is this vertical component that causes flexural motion and makes the acoustic wave switch sensitive to water. As a result of this sensitivity, when the shear acoustic wave switch is used in the presence of water, the level of which varies, such as when the switch is used outdoors in rain, the water can cause the switch to have a response that flickers. Although this flicker problem can be overcome by software processing as disclosed in the co-pending patent application entitled “Acoustic Wave Touch Detection Circuit and Method,” Ser. No. 10/454,003, filed Jun. 4, 2003, it is desirable to have an acoustic wave sensor and in particular an ice detector that is insensitive to water at any level so as to be able to detect the presence of ice alone.
It is noted that, besides the trapping of shear waves in a plate, it has been known that torsional waves could be trapped in a solid cylinder as described in the article “Trapped Torsional Modes In Solid Cylinders” by Ward Johnson, B. A. Auld and E. Segal, J. Acoust. Soc. Am. 100 (1), Jul. 1996. One application of a torsional mode trapped in a cylinder is an acoustic resonator for measuring force as shown in U.S. Pat. No. 5,813,280 to Johnson et al. However, the cylindrical body, in which the torsional wave is trapped, greatly limits the application and use of trapped torsional modes.